At an investigative hearing into the 787 Dreamliner battery’s design Tuesday, Boeing conceded its certification testing fell short and needs to be tightened in the future.
“In retrospect, we don’t believe it was conservative enough,” said Mike Sinnett, Boeing’s chief project engineer on the 787.
National Transportation Safety Board (NTSB) Chairwoman Deborah Hersman asked how a lithium-ion- battery failure, which wasn’t supposed to happen more than once in 10 million flight hours, could happen twice in just over 50,000 flight hours.
The testimony that came closest to an answer was that a test Boeing used to simulate an internal short circuit — by piercing a battery cell with a nail — failed to replicate the real-world outcome.
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said the nail test convinced engineers an internal short would cause overheating of a single cell but would not spread to other cells.
Hersman, in a phone interview after the hearing, said Boeing’s “assumptions and the testing were not as conservative as they could have been.”
In January, two apparent internal short circuits inside 787 batteries — one that caught fire in an empty plane on the ground in Boston and another that smoldered in flight in Japan — caused multiple cells to overheat.
Boeing in the future “may apply tighter test criteria … in areas of new technology.”
Still, he offered a stout defense of the overall battery design and testing process.
After an unprecedented three-month grounding of the new aircraft, the Federal Aviation Administration on Friday gave Boeing the go-ahead to install a revamped battery system in the 50 planes it has already delivered to customers.
Testimony on Tuesday, the first day of the two-day hearing, revealed Boeing made two major revisions to its design after battery-testing incidents prior to certification.
A serious fire that burned down a Tucson, Ariz., building owned by the maker of the 787 battery charger, Securaplane Technologies, prompted one set of improvements to prevent overcharging.
And a previously unpublicized 2009 incident, when a battery cell overheated in a test lab in Rockford, Ill., brought more design changes to protect against over-discharging.
Sinnett said the one-in-10-million-flight-hours calculation factored in the battery cell manufacturing, screening and quality-control processes, but did not include other possibilities, such as abuse of the batteries or installation of a
battery that doesn’t meet specifications.
He conceded that in the two overheating incidents in January, there was no evidence the batteries were abused or didn’t meet spec. The root cause of the incidents remains unknown.
Sinnett said it’s “too early to tell” if they were due to errors in the design or in the manufacture of the batteries.
He reiterated his conviction that “lithium-ion batteries are the right choice for the 787,” adding that — even without the latest battery fix — the 787 design includes protections if a battery fails.
In the in-flight incident in Japan, for example, when smoke was detected the air-conditioning system adjusted and vented the smoke out of the airplane.
“The overall system worked,” said Sinnett. “We didn’t have a catastrophic outcome. … The layers of protection performed their function.”
In the course of developing its fix for the battery system over the past three months, Boeing devised a new test — applying a heating element to one cell — that has replicated the propagation of overheating from cell to cell.
“It’s probably something we would use in the future as a standard,” Boeing systems engineer Jerry Hulm testified.
FAA representatives were also quizzed closely at the hearing about their original certification of the 787 battery system.
Ali Bahrami, head of the FAA’s Renton-based transport airplane directorate, said regulators were always aware of the risks from lithium ion batteries.
“We did the best we could under the circumstances, and the knowledge that existed, to come up with the standards” that Boeing’s battery had to meet, he said.
Dorenda Baker, director of the FAA’s aircraft certification service, emphasized that on a routine flight “these lithium batteries are not critical” to the jet’s safety.
“If a battery stops working in and of itself, it does not have much effect on the normal operation of the airplane,” she said.
And Steve Boyd, manager of the FAA airplane and flight-crew interface branch, defended the way the agency lets Boeing run all the testing needed for certification.
“The FAA as a matter of practice does not do its own technical evaluations,” he said. “Instead, we work closely with the manufacturers, reviewing the work that they have done. “
“Our own internal specialists … serve as an independent set of eyes on the manufacturer’s analyses and test plans,” Boyd said.
Also giving testimony Tuesday, all under oath, were representatives of Boeing’s battery system suppliers: Thales of France, the battery system integrator; and GS Yuasa of Japan, which manufactures the batteries.
Boeing’s Sinnett backed both suppliers to the hilt, saying that they were selected “through a very rigorous process” that included assessments of the “performance, history and capabilities” of each.
Dominic Gates: 206-464-2963 or firstname.lastname@example.org